Process for the manufacture of methyl cellulose ether

ABSTRACT

A water-soluble methyl cellulose ether, having a flocculation point below 100° C., is manufactured by reacting methyl chloride and a cellulose activated by mercerising the cellulose with aqueous alkali in the presence of a C 2 -C 3  alkyl chloride as a reaction medium at a temperature from 65° C. to 90° C. and at a pressure from 3 to 15 bar. The weight ratio between the cellulose and the C 2 -C 3  alkyl chloride is normally from 1:1 to 1:5. The use of the reaction medium makes it possible to produce at a low pressure methyl cellulose ether at a good rate.

The present invention relates to a process for the manufacture of amethyl cellulose ether by performing the methylation in the presence ofC₂-C₃ alkyl chloride as a reaction medium.

Methyl cellulose ethers are very common cellulose ethers and they canbesides the methyl substitution also contain other substituents, forinstance hydroxyethyl, hydroxypropyl and higher alkyl substituents. Ingeneral, the water-soluble methyl cellulose ethers have excellentwater-retaining properties and are suitable to use for example in cementand gypsum formulations in water-based paint formulations and inwallpaper pastes. Methyl celluloses of low viscosities are also used asa protection colloid in emulsion polymerisation of vinyl chloride.

The methylation is normally performed by reacting cellulose activatedwith aqueous concentrated NaOH and liquid methyl chloride at atemperature of about 70° C. The vapor pressure of methyl chlorideresults in a reactor pressure of about 20 bar or more. During thereaction the methyl chloride also reacts with NaOH and water and formsNaCl, methanol and dimethylether. To reduce the reaction pressure and toimprove the control of the methylation, the reaction can be performed inthe presence of a large amount of an inert water-soluble organicreaction medium, such as acetone, alkyl-blocked polyethylene glycols,ethylene glycol diethyl ether, isopropanol, tert. butanol ordimethoxyethane or mixtures thereof. After the reaction, the reactionmedium has to be removed from the methyl cellulose ether by processesthat often are complex and costly, such as extraction processes. Forinstance, U.S. Pat. No. 4,501,887 discloses the use of an inert organicreaction medium, which is an admixture of dimethoxyethane and at leastone further organic solvent selected from the group consisting ofalkanol, alkane diol and alkoxy alkanol. The amount used of theadmixture in the working examples for preparation of methyl celluloseethers is in relationship to the amount of cellulose very high, whichhas a negative influence on the yield. Furthermore, dimethoxyethane hasa boiling point of about 86° C. and forms an azeotrope with water atabout 80° C.

It is an aim of the present invention to provide a method, whichsimplifies the methylation and other steps in the production of methylcellulose ethers. According to the invention this can be achieved byreacting methyl chloride and cellulose activated by treatment withalkali in the presence of a reaction medium containing an alkylchloride, where the alkyl group is ethyl and/or a propyl group, at areaction temperature of 65-90° C. and a pressure of 3-15 bar, preferablyless than 10 bar. The weight ratio between the cellulose and thereaction medium is from 1:1 to 1:5, preferably from 1:1.3 to 1:2.5. Thepresence of a C₂-C₃ alkyl chloride and the limited reaction temperatureand low pressure makes it easy to control the methylation. Ethylchloride and propyl chloride have a boiling point of about 12° C. andabout 47° C. respectively, and they are easy to evaporate from thereaction mixture after completion of the reaction. Furthermore, theethyl chloride and propyl chloride are inert towards methyl chloride andhave a limited solubility in water. It has also been found that themethylation degree is high in comparison with the added amount of methylchloride, which also means that the formation of by-products, such asdimethyl ether and methanol are low. Furthermore, the clarity of theaqueous solutions of methyl cellulose ethers produced is excellent.

Besides the methyl groups it is also possible according to the presentinvention to introduce other substituents. Example of suitable othersubstituents are hydroxyethyl, hydroxypropyl, hydroxybutyl, ethyl,propyl and groups containing larger hydrocarbon groups, for examplesubstituents of the formula RO(C₂H₄O)_(n)CH₂CH(OH)CH₂—, where R is ahydrocarbon group with 4 to 24 carbon atoms and n is a number from 0 to7. Also ionic groups, such as carboxymethyl and groups containingprimary, secondary and/or tertiary amine or ammonium groups andquaternary ammonium groups can be present. Suitable reactants tointroduce the groups mentioned above are ethylene oxide, propyleneoxide, butylene oxide, ethyl chloride, propyl chloride,RO(C₂H₄O)_(n)CH₂CH(OH)CH₂Cl  (Formula I)or

where R and n have the meanings mentioned above, 2-chloro-acetic acid;3-chloro-2-hydroxypropyl trialkylammonium chloride and glycidyltrialkylammonium chlorides, where alkyl is an alkyl with 1-22 carbonatoms with the proviso that at most one of the alkyl groups contains1-22 carbon atoms; and the corresponding primary, secondary or tertiaryammonium and amine compounds. The reaction medium of the presentinvention may be used for all these reactants.

The activation of the cellulose can be achieved by mercerisation ofshredded cellulose with an aqueous concentrated NaOH solution containing40 to 55%, preferably 48 to 52%, by weight of NaOH. During the reactionNaOH is consumed by halogen-containing reactants such as methyl chlorideand ethyl chloride. Normally, the amount of NaOH is between 1.0 and 5.0moles per mole saccharide unit. The addition of the C₂-C₃ alkyl chloridecan take place before the mercerisation, during the mercerisation or indirect connection with the addition of methyl chloride but preferablynot after the addition of methyl chloride. The addition of C₂-C₃ alkylchloride before mercerisation may facilitate the activation of thecellulose. The methyl chloride can be added continuously,semicontinuously or in one step. Preferably the methyl chloride is addedgradually and the temperature raised to the reaction temperature of 65to 90° C., preferably 70 to 80° C. When the temperature has reached thedesired reaction temperature the remaining methyl chloride can be addedat such a rate that the pressure in the reactor is maintained at aconstant level.

As said above, the present invention also includes the possibility tointroduce other substituents in the methyl cellulose ether. Thus, theactivated cellulose may suitably be reacted with ethylene oxide and/orpropylene oxide before the addition of methyl chloride, or together withmethyl chloride for example in the initial phase of the addition ofmethyl chloride. The ethoxylation and propoxylation reaction willreadily take place between 50 to 80° C. and 60 to 80° C. respectively.Even a minor molecular substitution (MS) of hydroxyethyl and/orhydroxypropyl groups often has a positive effect on the yield in theassociated methylation as well as on the clarity of the methyl celluloseether solutions. The presence of hydroxyethyl substituents alsoincreases the flocculation temperature and balances the decreasing offlocculation temperature caused by a high substitution degree of methylgroups of the methyl cellulose ether. It may also be convenient tointroduce substitution of an ethyl or propyl group. This can easily bedone when the reaction with methyl chloride is nearly finalised byincreasing the temperature to above 85° C., suitable to 95 to 110° C.The increased temperature accelerates the methylation and at the sametime minor amounts of the reaction medium can react with the methylcellulose ether, whereby ethyl and/or propyl substituents are alsointroduced. The substitution degree mainly depends on the activation ofthe cellulose and the temperature and reaction time. The ethyl andpropyl substituents are rather hydrophobic and even a low degree ofsubstitution of these substituents evidently reduces the flocculationtemperature of the methyl cellulose. This reduction in flocculationtemperature may be of importance e.g. when the methyl cellulose ether iswashed with hot water. The reactant, chloro acetic acid, is normallyreacted with the activated cellulose at a temperature from 45° C. to 65°C., while reactions with a 3-chloro-2-hydroxypropyl trialkylammoniumcompound, a glycidyl trialkylammonium compound and the correspondingprimary, secondary and tertiary ammonium and amine compounds, and thereactants or formulae I and II are suitably performed at temperaturesfrom 45° C. to 110° C., depending on the size of the reactants. Thegeneral rule is, the smaller the reactant, the lower the reactiontemperature.

After completion of the reaction, the pressure is lowered and thereaction medium and other volatile components are driven off due to theheat of the reaction mixture. The obtained cellulose ether may be washedwith hot water above the flocculation temperature and cleaned fromwater-soluble salts and by-products in a conventional manner. The solidphase can be separated by centrifugation, dried to a desired moisturecontent (e.g. <3%) and grounded to a desired particle size (e.g. <1 mm).

The methyl cellulose ether of the present invention has normally aDS_(methyl) between 0.6 and 2.5, preferably between 1.0 and 2.0, aDS_(ethyl) between 0 and 0.8, a DS_(propyl) between 0 and 0.2, aDS_(carboxymethyl) between 0 and 0.2, a DS_(ammonium) or aminecontaining substituents between 0 and 1.0, aDS_(RO(C2H4O)nCH2CH(OH)CH2), where R and n have the meaning mentionedabove, between 0 and 0.5, a MS_(hydroxyethyl) between 0 and 1.6 and aMS_(hydroxypropyl) between 0 and 0.9 and a MS_(hydroxybutyl) between 0and 0.3. A methyl cellulose ether having no other types of substituentsthan methyl has suitably a DS_(metyl) between 1.0 and 2.0, preferablyfrom 1.2 to 1.8. Examples of mixed cellulose ethers are a methylhydroxyethyl cellulose ether having a DS_(metyl) from 1.2 to 2.0 and aMS_(hydroxyethyl) from 0.05 to 1.3, preferably from 0.1 to 0.7; a methylhydroxypropyl cellulose ether having a DS_(methyl) from 1.0 to 2.0 and aMS_(hydroxypropyl) from 0.1 to 1.0; a methyl ethyl hydroxyethylcellulose ether having a DS_(methyl) from 1.0 to 2.0, a DS_(ethyl) from0.1 to 0.6 and a MS_(hydroxyethyl) from 0.05 to 1.0; and a carboxymethylcellulose ether having a DS_(methyl) from 1.0 to 2.0 and aDS_(carboxymethyl) from 0.05 to 0.2.

The present invention is further illustrated by the following Examples.

EXAMPLE 1

In a reactor having a volume of 130 litre, 5.0 kg of milled cottonlinter was added, whereupon the gas phase of the reactor was evacuatedto 0.05 bar and filled with nitrogen gas. The evacuation and refill withnitrogen gas was repeated once and finally the reactor was evacuated to0.05 bar. At room temperature, 7.5 kg of ethyl chloride was then addedto the reactor during stirring and the mixture obtained was sprayed with6.17 kg of a water solution containing 50% by weight of NaOH (2.5 mol)to transfer the cellulose to alkali cellulose. During the mercerisationthe temperature raised to about 30° C. After 15 minutes of mercerisation0.27 kg of ethylene oxide (0.2 mol) was added followed by an addition of1.56 kg methyl chloride (1.0 mol).

The temperature was then gradually increased up to 75° C. during 30minutes, after which ethylene oxide and the essential part of methylchloride had reacted. Thereupon, at 75° C., an additional amount of 1.95kg methyl chloride (1.25 mol) was continuously introduced into thereactor in such a rate that the reactor pressure was kept on 8.5 bar.Without the presence of ethyl chloride the reaction pressure would havebeen above 19 bar.

After the addition of all methyl chloride the temperature was furtherincreased to 100° C. and a final reaction between ethyl chloride andremaining alkali was performed. The presence of ethyl groups in thecellulose ether leads to a decrease in the flocculation temperature andto improved dewatering ability.

After the reaction was completed the volatile components (including thereaction medium) were driven off and the rest of the reaction productwas washed by hot water about 95° C. to remove the salt formed duringthe reaction. The solid phase was separated by centrifugation and driedto a moisture content of less than 3% by weight. Finally the celluloseether was milled to a particle size less than 1 mm.

The cellulose ether was analysed with regard to its MS/DS: methyl, ethyland hydroxyethyl substitution by gas chromatography after cleavage byhydrogen bromide. The flocculation temperature was determined by use ofa spectrophotometer under a continuous temperature increase. Theviscosity was measured by a Brookfield viscometer, type LV, at atemperature of 20° C. in a 1% buffered solution at pH 7.0, while theclarity was measured by light transmission in comparison with water at20° C.

The degree of substitution of the different substituents and theproperties of cellulose ether are shown in the Table I below.

EXAMPLE 2

A cellulose ether was produced in a process similar to the one describedin Example 1, but with the differences that 7.41 kg of the watersolution containing 50% by weight of NaOH (3 mol), 1.36 kg ethyleneoxide and 2.34 kg methyl chloride (1.5 mol continuously) were added. Thedegree of substitutions and the properties of the cellulose etherobtained are shown in Table I below.

EXAMPLE 3

A cellulose ether was produced in a process similar to the one describedin Example 1, but with the difference that 1.56 kg of methyl chloridewas continuously added. The degree of substitutions and the propertiesof the cellulose ether obtained are shown in Table I below.

EXAMPLE 4

A cellulose ether was produced in a process similar to the one describedin Example 1, but with the differences that no ethylation was carriedout and that 7.41 kg of the water solution containing 50% by weight ofNaOH (3 mol) and 3.12 kg of methyl chloride (2 mol) in the continuousstep were added. The degree of substitutions and the properties of thecelulose ether obtained are shown in Table I below.

EXAMPLE 5

A cellulose ether was produced in a process similar to the one describedin Example 3, but with the difference that no ethylene oxide was added.The degree of substitution and the properties of the cellulose etherobtained are shown in Table I below.

EXAMPLE 6

A methyl cellulose ether was produced by a process similar to the onedisclosed in Example 1 by adding 2.34 kg methyl chloride (1.5 mol,continuously). No addition of ethylene oxide was made and no increase ofthe temperature to 100° C. was performed. The degree of substitution andthe properties of the cellulose ether obtained are shown in Table Ibelow.

EXAMPLE 7

In a reactor having a volume of 130 litre 5.0 kg of milled cotton linterwas added, whereupon the gas phase of the reactor was evacuated to 0.05bar and filled with nitrogen gas. The evacuation and refill withnitrogen gas were repeated once. After evacuation to 0.05 bar, 7.5 kg ofethyl chloride and 4.94 kg of a water solution containing 50% by weightof NaOH (2 mol) were added into the reactor during stirring at roomtemperature. After 15 minutes of mercerisation at 30° C., 1.36 kg ofethylene oxide (1 mol) was charged and the temperature was increased to60° C., whereupon 1.36 kg ethylene oxide (1 mol) was continuouslyintroduced during stirring for 20 minutes. The temperature was then keptfor 10 minutes at 60° C., and the temperature was then raised to 75° C.followed by the addition of 2.34 kg methyl chloride in such a rate thatthe reactor pressure was 8.5 bar. After the addition of all methylchloride the temperature was raised to 100° C. and ethyl chloride wasallowed to react with remaining NaOH. After completion of the ethylationthe reactor content was worked up as described in Example 1 and thecellulose ether analysed accordingly. The results are shown in Table Ibelow. TABLE I Example 1 2 3 4 5 6 7 Additions mole su¹⁾ NaOH 2.5 3.02.5 3.0 2.5 2.5 2.0 Ethylene 0.2 1.0 0.2 0.2 — — 1.0 + 1.0 oxide Methyl1.0 + 1.25 1.0 + 1.5 1.0 + 1.0 1.0 + 2.0 1.0 + 1.0 1.0 + 1.5 1.5chloride Substitutions DS ethyl 0.15 0.25 0.25 — 0.3 — 0.25 MShydroxyethyl 0.12 0.60 0.12 0.12 — — 1.20 DS methyl 1.3 1.5 1.2 1.7 1.21.35 0.85 Properties Viscosity, 12200 8830 8350 15900 14600 8900 5730mPa · s Clarity, % 92.6 95.9 88.4 90.4 77.5 81.2 96.1 Flocculation, 67.667.9 66.4 64.6 65.8 67.3 72.1 ° C.¹⁾su = saccharide unitFrom the results it is evident that methyl cellulose ether withdifferent types of substitutions can be produced according to theinvention at a low reaction pressure and low amounts of the reactionmedium. The cellulose ethers have good clarity and clarities above 90%are excellent. Further, the yield of methyl chloride is high incomparison with the results obtained in U.S. Pat. No. 4,501,887.

1. A process for the manufacture of a water-soluble methyl celluloseether, having a flocculation point below 100° C. which comprisesreacting methyl chloride and a cellulose activated by mercerising thecellulose with aqueous alkali, wherein the reaction is performed in thepresence of a C₂-C₃ alkyl chloride as a reaction medium at a reactiontemperature from 65° C. to 90° C. and at a pressure from 3 to 15 bar. 2.The process of claim 1, wherein the weight ratio between the celluloseand the C₂-C₃ alkyl chloride is from 1:1 to 1:5.
 3. The process of claim1, wherein the weight ratio between the cellulose and the C₂-C₃ alkylchloride is from 1:1.3 to 1:2.5 and the pressure below 10 bar.
 4. Theprocess of claim 1 wherein the C₂-C₃ alkyl chloride is added to thecellulose before the activation of the cellulose and/or during themercerisation of the cellulose.
 5. The process of claim 1 wherein aportion of the methyl chloride is added while the temperature is raisedto the reaction temperature, whereupon the remaining methyl chloride isadded in such a rate that the temperature and the reactor pressure ismaintained.
 6. The process of claim 1 wherein the methyl chloride isadded in an amount from 0.9 to 4.9 moles per mole saccharide unit of thecellulose.
 7. The process of claim 1 wherein the activated cellulose isreacted with ethylene oxide and/or propylene oxide at a temperature from50° C. to 80° C. and 60° C. to 80° C. respectively before the additionof methyl chloride and/or together with methyl chloride.
 8. The processof claim 1 wherein the activated cellulose is also reacted withreactants chosen from the group consisting ethyl chloride; propylchloride; butylene oxide; 2-chloro acetic acid;RO(C₂H₄O)_(n)CH₂CH(OH)CH₂Cl and

where R is a hydrocarbon having 4 to 22 carbon atoms and n is a numberfrom 0 to 7; 3-chloro-2-hydroxypropyl trialkylammonium chloride andglycidyl trialkylammonium chloride where alkyl is an alkyl with 1 to 22carbon atoms with the proviso that at most one of the alkyl groups havemore than 2 carbon atoms; and the corresponding primary, secondary ortertiary ammonium and amine compounds.
 9. The process of claim 1 whereinmethyl chloride and the other etherifying reactants are added in suchamounts that a methyl cellulose ether is obtained having a DS_(methyl)from 0.6 to 2.5, a DS_(ethyl) from 0 to 0.8, a DS_(propyl) from 0 to0.2, a DS_(carboxymethyl) from 0 to 0.2, aDS_(ammonium containing substituents) from 0 to 1.0,DS_(RO(C2H40)nCH2CH(OH)CH2), where R and n have the meanings mentionedabove from 0 to 0.3, a MS_(hydroxyethyl) from 0 to 1.6, a MShydroxypropyl from 0 to 0.9 and a MS hydroxybutyl from 0 to 0.3.
 10. Theprocess of claim 9, wherein the methyl cellulose produced is a methylcellulose having a DS methyl from 1.0 to 2.0; a methyl hydroxyethylcellulose having a DS_(methyl) from 1.2 to 2.0 and a MS_(hydroxyethyl)from 0.05 to 1.2; a methyl hydroxypropyl cellulose having a DS_(methyl)from 1.0 to 2.0 and a MS_(hydroxypropyl) from 0.1 to 1.0; a methyl ethylhydroxyethyl cellulose having a DS_(methyl) from 1.0 to 2.0, aDS_(ethyl) from 0.1 to 0.6 and a MS_(hydroxyethyl) from 0.05 to 1.2; anda methyl carboxymethyl cellulose having a DS_(methyl) from 1.0 to 2.0and a DS_(carboxymethyl) from 0.05 to 0.2.